Hydrogenation is an established process both in the chemical and petroleum refining industries. Hydrogenation is conventionally carried out in the presence of a catalyst, which usually comprises a metal hydrogenation component deposited on a porous support material. The metal hydrogenation component is often one or more metals, for example nickel, platinum, palladium, rhodium, ruthenium or mixtures thereof.
Many organic compounds have one or more groups or functionality that is susceptible to hydrogenation under appropriate conditions with the use of a suitable metal containing catalyst. One particular group of compounds that are susceptible to hydrogenation is those that contain one or more unsaturated groups or functionality such as for example carbon-carbon double bonds or triple bonds.
Hydrogenated derivatives of benzenepolycarboxylic acids or derivatives thereof, such as esters and/or anhydrides, have many uses. Of particular interest is their use as plasticisers for polymeric materials. In this context, the dialkylhexahydrophthalates are an example of one class of these compounds that are of particular interest. These materials may be produced by hydrogenation of the corresponding phthalic acid ester in the presence of a hydrogen-containing gas and an active metal hydrogenation catalyst deposited on a support.
Of particular importance in the hydrogenation of benzenepolycarboxylic acids or derivatives thereof is the degree of conversion of the starting materials and the selectivity of conversion into the desired hydrogenated cyclohexyl derivatives. The degree of conversion should be as high as possible and typically conversion levels of greater than 95% are sought and achieved for these types of hydrogenation. However, in these types of hydrogenation whilst high conversions may be obtained it is difficult to simultaneously achieve the required high degree of selectivity to the desired product. In this regard there is a problem with the generation of low molecular weight and/or boiling point by-products during the hydrogenation reaction. These by-products are often referred to as “lights” and they must be removed from the hydrogenation product before it is used, for example as a plasticiser.
The nature of the support material is often important to catalyst performance. Alumina is often used as a support in fields relating to petrochemical processing. However, it is sometimes preferable to use a nonacidic support, such as silica. U.S. Pat. No. 7,595,420 discloses a catalyst for hydrogenating benzenepolycarboxylic acids comprising platinum, palladium, ruthenium or mixtures thereof deposited on an ordered mesoporous support material having a high pore volume, a high surface area and controlled pore opening of at least 2 nm.
The arrangement of the metal on the support is also important. Metal particles should be small in size (i.e. the metal should be highly dispersed) and homogeneously distributed across the surface of the support. In order to maximize the number of available surface metal sites, the agglomeration of metal particles should be avoided.
A method of preparing supported metal catalysts is by reduction of a supported metal oxide. A typical method of making a supported metal oxide is by incipient wetness impregnation of a support with solutions containing metal salts, followed by drying and calcination. However, the high mobility of ruthenium oxide on silica makes the preparation of silica-supported ruthenium catalysts more challenging than the preparation of other silica-supported metal catalysts. In particular, the calcination of silica-supported ruthenium metal salts often leads to agglomeration of ruthenium oxide particles and results in a poorly dispersed catalyst.
Soled et al., St. Sur. Sci. & Cat., 162, 103-110 (2006) and US Patent App. Pub. 2006/166809 disclose a method of preparing silica-supported ruthenium catalysts that avoids the formation of ruthenium oxide intermediates. The process utilizes an amino alcohol or amino acid ligand, preferably triethanolamine, as a bifunctional dispersion aid. It is believed that ruthenium (amino alcohol/amino acid) complexes form strongly interacting precursors with silica support materials because the ligands simultaneously coordinate with both the support and ruthenium. Once the ruthenium complexes are dispersed across the support, the high hydrogenolysis activity of ruthenium is utilized to reduce and hydrogenolyse the ligands which would otherwise block active sites on the metal. However, a limitation of the process is the high temperature required for the hydrogenolysis step, typically about 400° C. Not all hydrogenation reactors, for example heritage hydrogenation reactors, are capable of reaching such temperatures, and so a complex external treatment step is often required.
Silica-supported rhodium catalysts are typically more straightforward to prepare because rhodium oxides are less mobile on silica than ruthenium oxides. Typically, the low mobility of rhodium oxides permits calcination of the catalyst precursor to remove organic ligands without the risk of metal particle agglomeration, and so there is no requirement for a high temperature hydrogenolysis step. However, a disadvantage of rhodium catalysts is the high cost of rhodium metal.
There is a need, therefore, for new hydrogenation processes for the conversion of benzenepolycarboxylic acids or derivatives thereof to the corresponding ring-hydrogenated derivatives, which processes produce lower levels of “lights” by-products and thus, offer improved selectivity for the desired products. There is also a need for new, efficient and cost effective supported metal hydrogenation catalysts having good metal dispersion and which can be prepared in a simple manner. It is therefore, an object of the present invention to provide a process for hydrogenating benzenedicarboxylic esters or anhydrides, using specific catalysts, by means of which the corresponding hydrogenation products may be obtained with high levels of conversion and selectivity.